This Week in Science

Science  01 Sep 2000:
Vol. 289, Issue 5484, pp. 1429
  1. Slip and Afterslip

    The magnitude 7.5 Izmit earthquake and the subsequent magnitude 7.1 Düzce earthquake were large strike-slip events on the North Anatolian fault in Turkey. This active fault has been studied intently by global positioning system satellites before and after these events, and Reilinger et al. (p. 1519) have modeled the amount and geometry of the subsurface rupture using this data. Large slip occurred in the upper 10 kilometers of the crust in distinct segments, whereas there was little slip at the hypocenter. After the Izmit earthquake, aseismic slip occurred near and below the hypocenter that may have triggered the subsequent Düzce earthquake.

  2. Superfluidic Molecular Hydrogen

    Molecular hydrogen can have its nuclear spins parallel (para-hydrogen) or antiparallel (ortho-hydrogen). It has been predicted that para-hydrogen should be superfluidic at low temperatures, just like helium-4. To reach the proposed superfluidic state, however, the liquid has to be extensively supercooled, which has hampered experimental observations. Grebenev et al. (p. 1532) used helium droplets as a cool, low-interference spectroscopic matrix, and observed superfluidity in small para-hydrogen clusters.

  3. Pushing Electrons the Hard Way

    Changes in a material's resistance in a magnetic field (magnetoresistance, or MR) has two parts—the tendency of the electronic orbitals to align in the magnetic field (physical MR) and the magnetic deflection of the electrons from their usually straight path (geometric MR). The former effect, characteristic of the layered manganites, or “giant” or GMR materials, usually dominates the geometric MR. Solin et al. (p. 1530) now show that geometric MR changes greater than five orders of magnitude can be achieved with relatively small magnetic fields using a simple structure: a loop of high-resistance, nonmagnetic indium antimonide with a low-resistance gold filling. In zero magnetic field, the electrons take the easy route through the gold, but a magnetic field forces the electron to take the high-resistance path.

  4. All Wet

    The South American tropics are suspected to have had a major effect on global climate since the last glacial maximum, but there is little agreement over how even one of the most basic climate signals, rainfall, has varied. Betancourt et al. (p. 1542) have constructed a new chronology of precipitation for the central Acatama Desert in Chile based on their surprising discovery of abundant remains of spring vegetation inside fossil rodent middens in what is now one of the driest places on Earth. Their record, supported by data on the rise and fall of the water table in nearby wetland deposits, differs in several important ways from other climate reconstructions for neighboring regions, and raises questions about what ultimately drives the long-term variability of the South American summer monsoon.

  5. Insert Atom Here

    One goal of chemical reaction dynamics is to see how the different rotational and vibrational states of a molecule influence its reactivity. Most of these studies have focused on simple abstraction reactions, such as H2 + F → H + HF, in which an incoming atom approaches a molecule in a linear geometry. Liu et al. (p. 1536) have examined the reaction of a particular atomic state of oxygen, O(1D), with para-H2, for energies at which the O atom inserts itself into the H2 bond to form OH and H in a “tee-shaped” geometry at high resolution. Although the overall reaction is symmetrical in the forward and backward directions, individual rotational and vibrational states can exhibit strikingly different angular distributions.

  6. Keeping Cholesterol in Check

    Controlling the absorption of dietary cholesterol from the digestive tract is an integral part of maintaining whole-body cholesterol homeostasis. Repa et al. (p. 1524; see the news story by Ferber) report that certain nuclear steroid receptors regulate this process. One nuclear receptor heterodimer stimulates the expression of a transmembrane protein of the ABC-binding cassette family and facilitates cholesterol efflux from intestinal cells. Another receptor heterodimer decreases the pool of bile acids in the liver, molecules that are needed to absorb free cholesterol. These activities serve to reverse cholesterol transport and decrease absorption of cholesterol and further expand the roles of nuclear steroid receptors in maintaining cholesterol homeostasis.

  7. Roll Out the Half Barrel

    The existence of similar domains in multidomain proteins indicates that they have evolved through gene duplication and fusion. Now Lang et al (p. 1546; see the Perspective by Miles and Davies) show that the single-domain β/α barrel likely evolved by duplication and fusion of the gene of a half-barrel ancestor. They determined the structures of two enzymes in the histidine biosynthesis pathway, HisA and HisF, at resolutions of 1.85 and 1.45 angstroms, respectively. Structural and sequence analysis strongly suggest that these two eightfold β/α barrels evolved from a common half-barrel ancestor. They suggest that an initial gene duplication would give two half-barrels that are then fused and adapted into an ancestral β/α barrel. A second gene duplication could lead to diversification into two enzymes with distinct catalytic activities.

  8. Anti-Aging Pills for Worms

    Reactive oxygen species, a normal by-product of cellular metabolism, are involved in the normal aging of cells and in age-related diseases. Melov et al. (p. 1567) have tested the effects of two small synthetic molecules that mimic the activities of superoxide dismutase and catalase (two enzymes known to “disarm” oxygen free radicals) on the longevity of adult worms. Growing worms in medium containing mimetics extended worm life-span by as much as 54%. The mimetics reversed the accelerated aging of worms carrying a mutation in the mev-1 gene encoding a mitochondrial electron transport protein.

  9. Nature's Cure for an Upset Stomach

    How does the gut, with its high load of microbes, normally avoid high levels of inflammation of its cellular lining? Neish et al. (p. 1560; see the Perspective by Xavier and Podolsky) show that certain bacteria present in the gut, upon binding to epithelial cells, can inhibit the movement of the pro-inflammatory protein NF-κB to the cell nucleus, where it would normally activate a cascade of genes leading to inflammation. This inhibition occurs indirectly by preventing the ubiquitination and, hence, the degradation of IκB-α, which inhibits the action of NF-κB by anchoring it firmly in the cytoplasm. The results provide insight into how host adaptations to the gut microflora benefit both host and microbe.

  10. Inhibited Just Enough

    The transcription factor NF-κB is a key mediator of the inflammatory response in mammals, and NF-κB inhibitors are of potential therapeutic value. A protein kinase complex containing the two IκB kinases IKKα and IKKβ and the regulatory protein NEMO (or IKKγ) causes activation of NF-κB in response to various inflammatory stimuli. (IκB is an inhibitor of NF-κB; when IκB is phosphorylated, its inhibition of NF-κB is relieved.) Thus, the IKKs are prime targets for therapeutic intervention, but inhibition of catalytic activity of kinases without affecting other similar enzymes can be difficult (or even harmful, because small amounts of NF-κB protect cells from apoptosis). May et al. (p. 1550) found that NEMO interacts with a very small region of the IKKs and that the interaction can be interrupted by a small peptide. When interaction of the IKKs with NEMO is disrupted, activation of NF-κB is effectively inhibited, but the beneficial basal activity of NF-κB is retained. The inhibitory peptide reduced inflammatory responses in two mouse models.

  11. No Weeds-But No Birds?

    Part of the debate surrounding genetically modified (GM) crops concerns the possibility of unanticipated environmental effects. Watkinson et al. (p. 1554; see the Perspective by Firbank and Forcella) present a model analysis of the potential impact of herbicide-tolerant crops on weed and bird populations. Their analysis consists of the linkage between a dynamic model of weed population at the field scale that includes parameters for crop management, a frequency distribution of weed levels across fields, and a model of the response of a bird species to these frequency distributions. The simulation of herbicide application results in severe reduction of weed populations that eventually affects the populations of seed-eating birds.

  12. A Special Sense of Smell

    The vomeronasal organ of mammals is a specialized structure in the nose that deals with the detection of pheromones. Little is known about how vomeronasal neurons react to natural stimuli and how they code their information for transmission to higher processing areas in the brain. Holy et al. (p. 1569) simultaneously recorded from a large number of mouse vomeronasal neurons and found that these cells responded to highly diluted components found in the urine of mice. Many individual neurons were highly selective for either male or female urine. Their activation required the recruitment of an intracellular signaling cascade involving phospholipase C. Unlike most sensory neurons, vomeronasal neurons did not adapt when exposed to a stimulus for a long time.

  13. Timing Differentiation

    The decay of radioactive isotopes can be used to trace the timing of the differentiation of Earth (the separation of the core, mantle, and crust into distinct chemical layers). Münker et al. (p. 1538) have estimated the timing of the formation of the terrestrial and lunar mantles using a relatively new isotopic system, niobium-92-zirconium-92. Their high-resolution measurements indicate that the terrestrial and lunar mantles formed about 50 million years after the formation of the solar system and a relatively long time after differentiation occurred on planetesimals and other smaller objects.

  14. All Features Big and Small

    The inverse relation between body size and population density in animal species is well established but only rarely has it been quantified accurately. Schmid et al. (p. 1557) now provide a survey, unrivaled in its detail, of body size and population density of several hundred species of invertebrates in two geographically separate European stream communities. Both communities, while sharing only a small proportion of species, have acquired similar density-size relations. They argue that the similarity of the distributions is brought about by the similarities in the physical properties of the two stream systems. [See the Perspective by Marquet.]

  15. RNA Editing and HIV Expression

    In RNA editing, the nucleotide sequence of the mature RNA molecule is altered from that of its genomic sequence. Multiple RNA editing events have been observed in various plant and animal cells and in viruses. Bourara et al. (p. 1564) now examine human immunodeficiency virus-type 1 (HIV-1) transcripts in virus producing cells. In addition to the C-to-U alteration that has been reported in other systems, a novel RNA editing event of G to A is seen. This G-to-A modification was only observed to be present in spliced messenger RNAs. Because the modifications noted here were found to be present in viral RNAs that are associated with survival of infected cells, this process may represent a control mechanism for HIV-1 expression.

  16. Does Rydberg State Manipulation Equal Quantum Computation?

    Ahn et al. (Reports, 21 January 2000, p. 463) reported results of experiments in which information was stored as quantum phase in a Rydberg atom data register, and showed that the information could be retrieved using a single laser pulse, in keeping with theoretical predictions. Meyer and Kwiat and Hughes note in separate comments that, because of unfavorable scaling characteristics, the system described by Ahn et al. cannot implement quantum algorithms more efficiently than can a classical digital computer, and thus cannot really be considered quantum computing. Kwiat and Hughes also suggest that the Ahn et al. system did not achieve quantum interference, and that their procedure thus did not accomplish inversion about the mean (IOM), a key feature in the search algorithms that the experiments sought to implement. Bucksbaum et al., in their response, acknowledge the scaling limitations of their setup and suggest some possible ways to overcome them in future experiments. They also disagree with the assertion by Kwiat and Hughes that interference “is not a general feature of these atomic wave packet experiments,” and argue that their procedure, irrespective of whether it followed the specifics of IOM, accomplished the same result. Notwithstanding unfavorable scaling for very large computational problems, they conclude, the Ahn et al. experiments “constitute progress that can…stimulate greater understanding and further developments in this field.” The full text of these comments can be seen at www.sciencemag.org/cgi/content/full/289/5484/1431a